The Smithsonian Hectospec Lensing Survey (SHELS) is a magnitude
limited spectroscopically complete survey for R ≤ 21.0 covering
4 square degrees. SHELS provides a large sample (15,513) of flux
calibrated spectra. The wavelength range covered by the spectra allows
empirical determination of k-corrections for the g- and r-band
from z = 0 to ~0.68 and 0.33, respectively, based on large
samples of spectra. We approximate the k-corrections using only two
parameters in a standard way: Dn4000 and redshift, z. We use Dn4000
rather than the standard observed galaxy color because Dn4000 is a
redshift independent tracer of the stellar population of the
galaxy. Our approximations for the k-corrections using Dn4000 are as
good as (or better than) those based on observed galaxy color (g-r)
(σ of the scatter is ~0.08 mag). The approximations for
the k-corrections are available in an on-line calculator. Our results
agree with previously determined analytical approximations from single
stellar population (SSP) models fitted to multi-band optical and
near-infrared photometry for galaxies with a known redshift. Galaxies
with the smallest Dn4000--the galaxies with the youngest stellar
populations--are always attenuated and/or contain contributions from
older stellar populations. We use simple single SSP fits to the SHELS
spectra to study the influence of emission lines on the
k-correction. The effects of emission lines can be ignored for
rest-frame equivalent widths (REWs) n4000 for ugriz and UBVRI (σ of
the scatter is typically ~0.10 mag). Again, the approximations
using Dn4000 are as good (or better than) those based on a suitably
chosen observed galaxy color. We provide all analytical approximations
in an on-line calculator at
this website.